There are a number of known ion sources for the elemental analysis of solids by mass spectrometry. Among these are "plain discharge sputtering" such as described in an article by Colburn et al., entitled "A new technique for elemental analysis of thin surface layers of solids", Applied Physics Letters, Vol. 19, No. 9, pg. 350, Nov. 1, 1971. According to Colburn et al., a sample is immersed in an RF sputtering glow discharge and a fraction of those sputtered atoms, which are ionized in the Penning discharge, are monitored with a quadrapole mass filter. In a letter appearing in Analytical Chemistry, Vol. 46, No. 3, March, 1974, beginning on page 461, Harrison et al. describe a modification of the Colburn et al. technique in which a hollow cathode tube is used as the ion source. In both of these techniques a self sustaining electrical discharge is used for the generation of ions. While this is adequate for most purposes, unfortunately the initial kinetic energy spread of the resulting ion beam is relatively high. Because of this the elemental ions must be analyzed with mass spectrometers capable of accepting a relatively high degree of energy spread. This necessity implies more expensive complex instruments than would be required with an ion beam having narrow energy spread.
Another problem encountered with some of these prior techniques is that the useable sputtering pressures are relatively limited since the hollow cathode has a rather limited pressure vs. discharge characteristic. This limits the versatility of the technique rather severely. Another problem encountered in these prior systems is that the sputtering rate is not easily controlled. This can limit the size of the sample, particularly if the surface being analyzed is a relatively thin one.
Accordingly, it is an object of this invention to provide an improved method for the elemental analysis of solids.
Another object of this invention is to provide an improved apparatus that facilitates the elemental analysis of solids.